Quasineutral　limit　for　a　model　of　three-dimensional　Euler-Poisson　system　in　half　space　with　a　boundary　layer　is　studied.　Based　on　the　matched　asymptotic　expansion　method　of　singular　perturbation　problem　and　the　elaborate　energy　method,　we　prove　that　the　quasineutral　regime　is　the　incompressible　Euler　equation.